The present invention relates to a drive system having one or more synchronous motors driving a common load, and in particular it relates to a power supply and control for electric inching of one or more three phase synchronous motors driving a grinding mill.
When a grinding mill is driven by synchronous motors, the running speed is fixed unless the motors are connected to a variable frequency power source. It is frequently desirable to run the grinding mill at speeds considerably slower than the normal running speed, for example, to inspect the mill liners when the mill is empty or to start the mill after it has been stopped with a full charge. This slow running is often referred to as inching or spotting.
Inching has been accomplished in several ways. One way involves the use of clutches. If the motors are coupled to the mill through clutches, the clutches may be partially engaged to cause rotation of the mill at lower speeds. This partial clutch closure for long periods generates considerable heat in the clutches and requires that wet clutches be installed and provision made to dissipate the heat generated. An installation using wet clutches is more expensive than one using dry clutches. Another way to provide for inching uses a removable hydraulic motor which is placed to engage the main mill pinion gears. This is inconvenient and requires a separate hydraulic motor. A third way to provide for inching uses a low frequency power source to provide power to the stator windings of the three phase synchronous drive motors. The low frequency power source may be a direct current (DC) supply connected to an inching supply bus for the motors through a series of electro-mechanical or static switches to produce stepped low frequency three phase voltages. These switches are referred to as sequencing or commutating switches. If there are a number of grinding mills at one location, only one such supply is required and it can be switched to a particular mill as desired. The present invention is concerned with this last way of providing inching.
Technical Information bulletin GET-1722C, E.A.E. Rich, entitled "Spotting Equipment for Synchronous Motors", published November 1966 by the General Electric Company, describes in some detail the inching or spotting of synchronous motors using a direct current supply and a pair of switches or contactors for each phase whereby a phase winding can be connected to the positive side of the supply, to the negative side of the supply, or not connected. By sequencing the switches in a predetermined manner, a magnetic field is created by the stator windings which slowly rotates or steps causing the rotor to rotate in steps. Reference to this bulletin will provide further detail of this way of providing inching. The textbook "Power Semiconductor Circuits" by S. B. Dewan and A. Straughen, 1975, a Wiley-Interscience Publication (ISBN 0-471-21180-X) about page 425 shows a stepped voltage waveform that can be obtained from a converter.
When a mill is being inched with no load or ball charge in the mill, for example, to inspect the mill liners, then the torque required is relatively constant and the torque is less than required for normal running. When a loaded mill is being inched, the rotation begins from a rest position where the charge is at the bottom. The torque initially required to begin rotation is relatively small. As the mill is rotated or inched to the cascade position where the charge starts to tumble, the torque required increases quite considerably as the charge is moved away from the rest position on a large radius. The cascade position may be around 40 to 50 mechanical degrees although it is sometimes greater. Once the charge begins to tumble the required load torque drops. It will be seen that it is very desirable to have some control over the developed torque.
If the developed motor torque matches the load torque plus the friction torque, then the rotation will be continuous and smooth. If the developed torque is not sufficient the rotor will slip poles. If the developed torque is much larger than the load torque required, the rotor will try to move at excessive speeds to the next position, i.e., it will jump to the next position, causing vibration and placing considerable strain on the gear teeth coupling the motor to the mill. This strain can result from several situations or conditions during inching.
For example, when a DC supply is commutated by switching to develop a low frequency stepped alternating supply for the stator (at perhaps, for example, one hundredth of the normal operating frequency) to inch the motor, the stator supply becomes a succession of discrete steps approximating a low frequency sinusoidal waveform. The torque developed by the motor results from the slowly rotating stator field interacting with the rotor field. Thus the rotor tends to advance in discrete steps. If the developed torque is much larger than the load torque plus the friction torque, the rotor will jump to, and oscillate about, each new position as the field rotates in steps and this will cause severe vibration and "hammering" of the gears. It is difficult to avoid the vibration and the resulting strain on the gears, as during inching the load torque increases from a relatively low level as the mill leaves its rest position to a relatively high level just before the cascade position. It is difficult to match the developed torque to the changing load torque. Copending U.S. application Ser. No. 556,906 (continuation-in-part of Ser. No. 448,316 now abandoned)--Eastcott et al filed Dec. 1, 1983 and assigned to Canadian General Electric Company Limited, describes one way of reducing and controlling this vibration.
Another example of a situation which can provide gear strain occurs when the load in the mill tumbles and the load torque is suddenly reduced. If the developed motor torque remains at the same level there will be vibration and hammering as the inching continues.
Referring for the moment to FIG. 1, there is shown a graph of reference voltage plotted against mill rotation as might appear when inching a load such as a loaded grinding mill. The graph would also represent torque plotted against mill rotation as the torque is approximately proportional to the voltage. The load torque is represented by curve 1. As the mill begins to rotate there is an initial torque required to overcome friction and start the rotation of the mill. The torque requirement then decreases slightly, then begins to increase as the mill rotates and raises the load which had settled at the bottom when the mill was stopped. The torque continues to increase as the load is rotated farther away from the bottom position it had when the mill was stopped. When the load in the mill cascades (about point 2 on the curve) the torque requirement decreases until a substantially steady state is reached.
In the past it has been known to provide a manually adjustable reference for the DC supply. An operator could set the voltage required to provide a maximum torque as required by mill cascade conditions. This is represented by the broken line 3 in FIG. 1. This reference could not follow changing conditions, although it was adequate for an experienced operator when inching an empty mill.
Subsequently an improvement was devised where the reference controlling the DC level was ramped up to a maximum to approximate increasing load torque as a loaded mill was inched from rest position to cascade position. This is represented by curve 4 in FIG. 1. It will be apparent that the sloped portion or ramp portion of curve 4 must reach the steady or constant level before maximum load is reached, that is before the load is expected to cascade. Depending on the gear ratios and the motors, this ramp portion might be associated with, for example, a time of two to three minutes after inching is started. As the load in a mill cannot in practice be determined with sufficient accuracy before inching, and as the cascade position varies, it is difficult to provide an accurate ramped reference and to reduce that reference as soon as the load tumbles. The aforementioned U.S. application Ser. No. 556,906 reduces vibration by detecting the vibration with a sensor and reducing stator voltage to hold the vibration to an acceptable minimum. While this provides simple and adequate control, the response is sometimes not able to compensate rapidly enough for sudden changes in torque. The curve 5 in FIG. 1 represents the torque provided according to the invention where the ramped reference 4 is reduced by the control of this invention to provide torque in accordance with curve 5.